The present invention relates to sensors for detecting falling hail and, more particularly, to such sensors employing piezoelectric transducers.
Sensors and sensing systems for detecting falling precipitation are known in the art. The simplest systems for detecting rain contain an alarm circuit with a switch that is actuated by falling rain. For example, U.S. Pat. No. 1,123,012 discloses a rain sensor containing such a circuit with a spring loaded switch that is held open by a string. A chemical substance, such as calcium phosphide, which immediately takes fire when becoming wet by the falling rain, burns the string allowing the switch to close and activate the alarm. Other rain indicators, such as U.S. Pat. No. 1,137,674, contain a pivoting switch which is closed when rain is collected in a container mounted on the switch. In U.S. Pat. No. 2,443,687, the contacts of a spring loaded switch are separated by a water soluble wafer. When falling rain wets the wafer, it is crushed by the spring loaded contacts thereby closing the electric circuit and energizing the alarm.
Devices for detecting when snowfall reaches a predetermined height have also been developed. U.S. Pat. No. 3,530,453 discloses a snowfall alarm which comprises a photocell and light source cooperatively mounted on opposite sides of a vertically adjustable table on which the falling snow accumulates. When snow accumulates to the height of the light beam between the photocell and light source, an alarm is sounded.
Electrodes or windings that are bridged by the accumulation of falling precipitation have also been used to activate an alarm circuit. The electrodes generally consist of two parallel disposed electrically conductive screens connected in series. Moisture which bridges the small gap between the two screens completes the circuit and thereby energizes an alarm or other equipment. Examples of such systems can be found in U.S. Pat. Nos. 2,717,957, 3,408,477, 3,157,800, 4,068,133 and 4,656,333.
Other precipitation detectors, such as U.S. Pat. No. 3,575,641, contain heating-sensing elements, such as thermistors, in a bridge circuit. When one of the thermistors is exposed to the falling precipitation and is cooled, the imbalance produced in the bridge circuit is conveyed to a relay which activates a load. When the precipitation stops, the exposed thermistor will warm up and balance the bridge circuit. The relay will then close to deactivate the load.
Systems have also been developed to detect and indicate the presence of ice formed on the surfaces of aircraft, roads, bridges and other vehicles. U.S. Pat. No. 2,419,454 discloses the use of a magnetostriction or piezoelectric device which emits sound waves along a surface where ice may accumulate. If a layer of ice is present on the surface, some of the emitted sound waves are reflected toward a receiver where a reading, proportional to the ice thickness, is displayed on a meter. If the surface is free of ice, none of the sound waves are reflected toward the receiver and a zero reading is displayed on the meter.
Other ice detectors, such as that disclosed in U.S. Pat. No. 3,341,835, employ a magnetostriction or piezoelectric vibrator whose resonant frequency changes as ice builds up on its exposed portions. U.S. Pat. No. 3,706,981 discloses an ice detection system with a piezoelectric crystal whose amplitude of vibration becomes damped as ice forms on the crystal. This damped vibration is detected by the system's circuitry and an appropriate output signal is produced to indicate ice formation.
Although there are various devices for detecting precipitation and the presence of ice on a surface, a need has developed for a sensor which only detects the presence of falling hail. These sensors could be strategically positioned around airports so that aircraft can be guided around areas experiencing a hailstorm.